The present invention relates to the field of measurement tools, e.g., suitable for use in equipment for oil prospecting and production.
More specifically, after a well has been bored, that type of activity requires sondes or sensors, in particular electrical or electromagnetic sondes or sensors to be inserted into the hole to enable measurements to be performed serving to characterize, amongst others, which fluids are present in the terrain and layers around the borehole, and also the dip of said layers. The term xe2x80x9cloggingxe2x80x9d is used to designate any continuous recording as a function of depth of variations in a given characteristic of the formations around a borehole.
One of the characteristics that it is important to know in a borehole is the resistivity of the drilling mud used. The resistivity of the mud is a parameter that is used, in particular, to correct measurements relating to other characteristics of the surrounding formations. In order to discover this mud resistivity, various approaches are already known.
In a first approach, mud resistivity is measured by a device that requires additional equipment on the tool already used for measuring the characteristics of the formation, which additional equipment may be, for example, of the AMS type (described in document EP013 224). That technique gives rise to additional costs and to apparatus that is of greater bulk.
In another technique, the resistivity of the mud is measured at the surface from a fluid sample. Extrapolation then makes it possible to take account of temperature dependence relative to downhole conditions by measuring the temperature down hole. The accuracy obtained is often unsatisfactory, essentially for the following two reasons:
difficultly in obtaining an accurate measurement of the temperature downhole; and
the characteristics of the fluid in the borehole can change with depth, in which case the sample available on the surface is no longer representative.
An object of the invention is to provide a novel method and novel apparatus enabling a measurement to be obtained of the resistivity of the mud in a borehole, without requiring additional specific apparatus to be implemented, but capable of making use of electrode structures that already exist. In addition, the new method and the new apparatus must be capable of measuring the resistivity of the mud in situ, without it being necessary to take samples for subsequent analysis on the surface. Finally, it is desirable to find a method and an apparatus that enable measurements to be made on the mud without requiring any prior measurement of the azimuth resistivity of the surrounding formations, and which is relatively insensitive to the influence of the diameter of the borehole.
In a first aspect of the invention, the invention provides a method of measuring the resistivity Rm of a drilling mud inside a borehole passing through a terrestrial formation, the method comprising:
inserting a sonde into the borehole, the sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode;
emitting at least one current I0 into the surrounding formation from the annular current electrode;
focusing the current I0 in the formation by emitting two currents I1 and Ixe2x80x21 from the annular guard electrodes situated on either side of the annular current electrode; and
producing a signal in response to the emitted current I0, which signal is representative of the resistivity Rm of the drilling mud.
This method is a method of measuring the resistivity of the mud in situ. It does not require any prior knowledge of the azimuth resistivity of the surrounding formations. In addition, it is relatively insensitive to the effects due to variations in the dimensions of the borehole, particularly when the borehole diameter is relatively large. Finally, it should be observed that mud resistivity is measured by emitting current into the surrounding formation, and not by emitting surface current into the mud.
A signal may be produced that is representative of a voltage induced through the borehole mud by the current I0 circulating through said mud and the formation.
The sonde may include a single annular current electrode, first and second pairs of annular electrodes referred to as electrodes for measuring voltage in the borehole mud, each pair being disposed on either side of the annular current electrode, the resistivity Rm being deduced from the ratio (V1xe2x88x92V3)/I0 in which V1 and V3 are the mean potentials of the two pairs of electrodes for measuring voltage in the drilling mud.
In another embodiment, the sonde may include:
two annular current electrodes respectively emitting a current I0 and a current Ixe2x80x20 into the surrounding formation; and
an annular potential-measuring electrode situated between the two current electrodes or else an array of azimuth electrodes situated between the two annular current electrodes.
This embodiment is particularly well adapted to enabling the method to be implemented using electrode structures that already exist.
The invention also provides an apparatus for measuring the resistivity of drilling mud in a borehole passing through a terrestrial formation, the apparatus comprising:
a sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode;
means for emitting at least one current I0 into the surrounding formation from the annular current electrode;
means for focusing the current I0 in the formation by emitting two currents I1 and Ixe2x80x21 from the two annular guard electrodes situated on either side of the annular current electrode; and
means for producing a signal in response to the emission of the current I0, said signal being representative of the resistivity Rm of the drilling mud.
This apparatus is associated with the same advantages as those specified above with reference to the first method of measurement of the invention: it enables measurements to be performed in situ, and it does not require prior knowledge of azimuth resistivities.
The apparatus may include means for producing a signal representative of a voltage induced through a drilling mud by the current I0, because of the current flowing through the mud and through the formation.
Thus, the sonde may include a single annular current electrode, first and second pairs of annular electrodes for measuring voltage in the drilling mud, each pair being disposed on either side of the annular current electrode, the means for producing a signal representative of the resistivity Rm enabling Rm to be deduced from the ratio (V1xe2x88x92V3)/I0, where V1 and V3 are the mean potentials of the two pairs of electrodes for measuring voltage in the drilling mud.
In another aspect, the same apparatus may be such that the sonde includes:
two annular current electrodes;
means for emitting into a surrounding formation a current I0 via one of the annular electrodes, and a current Ixe2x80x20 via the other annular electrode;
an annular electrode for measuring potential, situated between the two current electrodes, or else an array of azimuth electrodes situated between the two annular current electrodes.
The tools of the prior art, and those described above, require the current I0 or the current I0 and Ixe2x80x20 as emitted from the annular current electrode(s) into the terrestrial formation to be focused. Means must therefore be implemented for providing such focusing. In general, this requires a feedback loop to enable the focusing current(s) to be adjusted as a function, for example, of a signal representative of a focusing potential. In theory this implies amplification with infinite gain, but in practice gain must be limited in order to ensure stability. In particular, when using focusing potential measurement electrodes, as is usually the case, the result is that these electrodes are not at exactly the same potential and this gives rise to a measurement error. Although the error is very small, particularly in standard xe2x80x9cDual Laterologxe2x80x9d type tools, it can become large when the spacing between the focusing voltage measurement electrodes is reduced in order to improve the resolution of the apparatus.
Consequently, it is desirable to be able to propose a method and apparatus for measuring the resistivity of drilling mud that enable the objects already specified above to be achieved while also making it possible to eliminate errors due to the presence of a feedback loop.
The invention thus also provides a method of measuring the resistivity of drilling mud in a borehole passing through a terrestrial formation, the method comprising:
inserting a sonde into the borehole, the sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode;
performing computed focusing to simulate an operating mode in which:
at least one current I0 is emitted into the surrounding formation from the annular current electrode;
the current I0 is focused in the formation by emitting two currents I1 and Ixe2x80x21 from the two annular guard electrodes situated on either side of the annular current electrode;
producing a signal representative of the resistivity Rm of the drilling mud from the simulated operating mode.
This method does not require any direct focusing to be implemented, and it makes use only of focusing by computation. Since the stimulation is generally performed by computer apparatuses on the surface, the measurement tool as used is considerably simplified. Also, insofar as no direct focusing takes place during measurement, the means for controlling and/or regulating the focusing current are not implemented. This avoids all of the focusing current feedback loops.
In addition, this method does not require prior knowledge of the azimuth resistivity of the surrounding formations. It is less sensitive than prior art methods to effects due to variations in the dimensions of the borehole.
In a particular implementation, the computed focusing may be performed on the basis of two real or xe2x80x9ceffectivexe2x80x9d operating modes of the sonde:
a first mode in which current having great penetration depth is emitted into the surrounding formations; and
a second mode in which current having shallow penetration depth is emitted into the surrounding formations.
In the first mode, the currents of greater penetration depth subsequently return to the surface. In contrast, in the second mode, the currents do not penetrate very far into the surrounding formations.
The computed focusing may be implemented on the basis of the two following modes:
a first operating mode in which current is emitted into the surrounding formation, specifically a current i1 from one of the annular guard electrodes and a current ixe2x80x21 from the other annular guard electrode, the current emitted by the annular current electrode(s) being equal to 0;
a second operating mode in which at least one current i0 is emitted from the annular current electrode(s) to the annular guard electrodes, with the total current emitted from the sonde into the formation being equal to 0.
In each mode, signals may be produced that are representative of a xe2x80x9cfocusingxe2x80x9d voltage and of a xe2x80x9csondexe2x80x9d voltage; in addition, in the second mode, a signal may be produced that is representative of the current emitted from the current electrode(s).
In one computation technique, it is possible to deduce a weighting coefficient from a linear combination of the two effective operating modes of the sonde so as to obtain a computed mode for which the resultant focusing voltage is zero.
In another computation technique, a signal is also produced in the first mode that is representative of the total current emitted into the formation, and transfer impedances or coefficients are calculated between:
firstly the focusing voltage and the sonde voltage; and
secondly the current emitted from the current electrode(s) and the total current emitted into the formation.
The measured value of Rm may then be deduced from the ratio of the sonde voltage value divided by the current value emitted from the current electrode(s), for which values the focusing voltage is zero.
The sonde may comprise:
a single current electrode;
first, second, and third pairs of potential-measuring electrodes disposed on either side of the current electrode;
the focusing voltage being equal to the difference V1xe2x88x92V2 between the mean voltages from the first and second pairs of potential-measuring electrodes;
the sonde voltage being equal to the difference V2xe2x88x92V3 between the mean voltages from the second and third pairs of potential-measuring electrodes.
In a variant, the sonde may comprise:
two annular current electrodes; and:
either an annular potential electrode disposed between the two current electrodes;
or else an array of azimuth electrodes disposed between the two current electrodes;
and first and second pairs of annular potential-measuring electrodes;
the focusing voltage being equal to the difference between the mean voltage of the first pair of annular potential-measuring electrodes and either the voltage of the annular potential electrode disposed between the two current electrodes, or the mean voltage of the array of azimuth electrodes;
the sonde voltage being equal to the difference between the mean voltages of the first and second pairs of annular potential-measuring electrodes.
The invention also provides an apparatus for measuring the resistivity of drilling mud in a borehole passing rough a terrestrial formation, the apparatus comprising:
a sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode;
means for performing computed focusing so as to simulate an operating mode in which:
at least one current I0 is emitted into the surrounding formation from the annular current electrode;
the current I0 is focused in the formation by emitting two currents I1 and Ixe2x80x21 from the annular guard electrodes situated on either side of the annular current electrode;
means for computing a signal representative of the resistivity Rm of the drilling mud on the basis of the simulated operating mode.
This apparatus does not require means to be implemented for providing effective control of focusing current. It therefore avoids any feedback loop. In addition, it makes it possible to implement the abovedescribed method, with all of the corresponding advantages.
The sonde may also include means for use in a first effective operating mode to emit currents of great penetration depth into the surrounding formations, and in a second effective operation mode for emitting currents of small penetration depth into the surrounding formations, with the means for performing computed focusing performing the focusing on the basis of these two effective modes of operation.
Thus, the sonde may comprise:
means for emitting into the surrounding formation in a first effective operating mode both a current i1 from one of the annular guard electrodes and a current ixe2x80x21 from the other annular guard electrode, the current emitted from the annular current electrode(s) being equal to 0;
means for emitting, in a second effective operating mode, at least one current i0 from the annular current electrode(s) to the annular guard electrodes, the total current emitted from the sonde into the formation being equal to 0;
the means for performing computed focusing operating on the basis of these two effective operating modes.
Means may be provided to produce:
signals representative of a focusing voltage and of a sonde voltage;
a signal representative of the current(s) emitted from the current electrode(s).
In order to implement a first computation technique, in a first embodiment, the means for performing computed focusing enable a weighting coefficient to be deduced from a linear combination of the two effective operating modes of the sonde, and to obtain a computed mode for which the resultant focusing voltage is zero.
In order to implement another computation technique, in another embodiment, means may be provided for use in the first effective operating mode to produce a signal representative of the total current emitted into the formation, the means for performing computed focusing serving to deduce transfer impedances or coefficients between:
means being provided for producing in the first effective operating mode, a signal representative of the total current emitted into the formation;
the means for performing computed focusing enabling transfer impedances or coefficients to be deduced between:
firstly the focusing voltage and the sonde voltage; and
secondly the current emitted from the current electrode(s) and the total current emitted into the formation.
The means for computing a signal representative of the resistivity Rm may be suitable for deducing Rm from the ratio of the sonde voltage value divided by the value of the current emitted from the current electrode(s), for which values the focusing voltage is zero.
The methods described above may also include a step of correcting the measured values Rm to take account of the following sources of error
the highly resistive nature of the surrounding formation;
the presence of one or more highly conductive beds in the formation;
the influence of the borehole.
These corrections may, for example, be implemented by means of an extended Kalman filter.
The corresponding apparatuses may include corresponding means for implementing the corrections.